U.S. Pat. No. 5,515,829 to Wear et al. describes a variable displacement actuating fluid pump for a hydraulically-actuated fuel injection system. In this system, a high pressure common rail supplies pressurized lubricating oil to a plurality of hydraulically-actuated fuel injectors mounted in a diesel engine. The common rail is pressurized by a variable displacement swash plate type pump that is driven directly by the engine. Pressure in the common rail is controlled in a two-fold manner. First, some pressure control is provided by electronically varying the swash plate angle within the pump. However, because variable angle swash plate type pumps typically have a relatively narrow band of displacement control, pressure in the common rail is primarily controlled through an electronically controlled pressure regulator. The pressure regulator returns a portion of the pressurized fluid in the common rail back to the low pressure fluid sump in order to maintain fluid pressure in the common rail at a desired magnitude.
While the Wear et al. hydraulically-actuated system using a variable displacement pump has performed magnificently for many years in a variety of diesel engines manufactured by Caterpillar, Inc. of Peoria, Ill., there remains room for improvement. For example, variable angle swash plate type pumps are relatively complex, and thus are more prone to mechanical break down relative to simple fixed displacement type pumps. In addition, the Wear et al. system inherently wastes energy that inevitably results in a higher than necessary fuel consumption for the engine. In other words, energy is wasted each time the pressure regulator spills an amount of pressurized fluid back to the low pressure sump in order to control rail pressure. The Wear et al. system primes itself by having its pump biased to produce substantial output, even when system pressures are low, such as during a cold start. The Wear et al. pressure regulating valve and/or a separate pressure relief valve provide the means by which system over pressurization is avoided.
The present invention is directed to overcoming problems associated with, and improving upon, hydraulic systems.